1. Field of the invention
This invention relates to the protection of a power semiconductor device which controls the power applied to either an A.C. or D.C. driving motor of an electric rolling stock or train.
2. Description of the Prior Art
Recently, a power semiconductor device, such as chopper equipment or inverter equipment, comprising power semiconductors has been employed for driving electric rolling stock 1, such as a train, in order to reduce power consumption and maintenance cost and to improve the performance.
FIG. 1 shows one example of the main circuit in such an inverter equipment for an A.C. drive motor. In FIG. 1, reference numeral 1 designates a current collector for collecting direct current applied, for instance, to a power line 1a from a power source, such as a substation (not shown); 2, a first switch; 3, a second switch; 4, a charging resistor for a filter capacitor 6; 5, a filter reactor forming an inverted L-shaped filter circuit with the filter capacitor 6; 7, an inverter power control means or circuit comprising semiconductor thyristors; and 9, a three-phase induction motor which is the main electric motor for driving electric rolling stock such as a train. The aforementioned circuit elements 2 through 7 form an inverter equipment 8.
The operation of the circuit thus organized will now be described. When an operation instruction is applied, for instance, by a train operator, the first switch 2 is closed, so that the filter capacitor 6 is charged through a circuit consisting of the current collector 1, the first switch 2, the charging resistor 4 and the filter reactor 5. In this case, the power control means or circuit 7 is not in operation, and, therefore, no current flows in the power control means 7 and induction motor 9. After a lapse of a predetermined period of time at which the filter capacitor 6 has charged up, the second switch 3 is closed to short-circuit the charging resistor 4. When the second switch 3 is turned on in this manner, the power control means 7 starts its operation to convert D.C. voltage into three-phase A.C. voltage to control the three-phase induction motor to drive the electric rolling stock.
One example of the sequences for closing the first and second switches 2 and 3 will be described in more detail with reference to FIG. 2 in which is shown a control circuit to be used for the inverter equipment in FIG. 1. In FIG. 2, the operation instruction 19 issued from an operation equipment (not shown) operated by the train operator is applied to an electromagnetic valve 20 provided in association with the first switch 2, thereby exciting the electromagnetic valve 20. Upon energization of the same, the first switch 2 is closed and an auxiliary contact 21 of the first switch 2 is closed. Upon closure of the auxiliary contact 21, an electromagnetic valve 23 provided in association with the second switch 3 is excited due to an application of a voltage on a power line 22 connected to a D.C. source for the control circuit, and the second switch 3 is closed. When the second switch 3 is closed, its auxiliary contact 24 is closed, whereby a power on signal 26 is inputted to a control circuit 25 for controlling the power control means 7. In response to both the power on signal 26 and the operation instruction 19, the control circuit 25 instructs the power control means 7 to start its operation.
The resistance of the charging resistor 4 has been set to a value R with which, during the charging operation, the voltage of the filter capacitor 6 is not oscillated by resonance of the filter reactor 5 and the filter capacitor 6. If an inductance L of the filter reactor 5 is set to 12 mH, a cpacitance C of the filter capacitor 6 to 3,600 F. and a resistance R of the charging resistor 4 to 3.8, then the resistance R of the charging resistor 4 needs to meet the following relation in order to suppress an LC resonance: ##EQU1## Putting the values of L and C into the above, ##EQU2## It can be appreciated that the given values of L, C and R meet the above relation (1), thus the LC resonance can be suppressed.
The time required for charging up the filter capacitor 6 is determined by the inductance L of the filter reactor 5, the capacitance C of the filter capacitor 6 and the resistance R of the charging resistor 4. A charging time constant of the filter capacitor 6 is caluculated by: EQU =C.multidot.R=13.7 msec. (3)
Since it takes about 100 msec for the second switch 3 to be closed starting from the exciting of the electromagnetic valve 23, the charging of the filter capacitor 6 has already been completed when the second switch 3 is closed. The timing chart for explanation of the above relation is shown in FIG. 3.
FIG. 4 is a circuit diagram showing one example of the power control means 7 for controlling an A.C. drive motor 9. In FIG. 4, reference numerals 7U through 7Z designate gate-turn-off type thyristors in the arms of an inverter. The order of firing of the thyristors 7U through 7Z is controlled by gate signals to obtain three-phase A.C. output voltage from D.C. input voltage. The control operation will not be described here, it being well known in the art.
FIG. 5 shows one example of the main circuit of a chopper equipment. In FIG. 5, reference numeral 10 designates a chopper power control means comprising semiconductor thyristors; 2 through 6, the same circuit elements as those in FIG. 1; 12, a free wheeling diode; 13, a main D.C. driving motor having an armature 14 and a field system 15; and 16, a main smoothing reactor. The circuit elements 2 through 6 and 10 form the chopper equipment 11.
Similarly as in the case of the inverter equipment of FIG. 1, when an operation instruction is applied to the chopper equipment, for instance, by the operator, the first switch 2 is turned on. Then, when the second switch 3 is closed, the chopper power control means 10 starts the chopping operation, to convert D.C. voltage into variable continuous voltage, to control the main D.C. motor 13.
The free-wheeling diode 12 is provided for the reflux of a current to be frown into the chopper power control means 10 when the latter becomes off. The current flowing in the diode 12 gradually decreases.
Two typical conventional equipments have been described. In such conventional equipments, if the main circuit is grounded, for instance, by defective semiconductor elements upon closure of the second switch 3, a ground-leading current flows through a circuit consisting of the current collector 1, the first switch 2, the second switch 3, the filter reactor 5 and the power semiconductor device 7 or 10 from the power line 1a, as a result of which an over-current detecting means located, for instance, in the substation supplying electric power to the electric rolling stocks or trains, would be operated for safety and security. The main circuit is grounded if, for instance, the thyristors 7U and 7X in FIG. 4 simultaneously failed. In such a case, the supply of electric power to the power line la is interrupted, and, therefore, no electric power is supplied to other trains running on lines served by the same power source.